From the sounds of it, i would go with the GP. No matter how much money you put into a cavi, it'll need more in a month. And since the GP is related to the bonneville motor wise, that is all good. And judging by the deal she is getting, it seems like a good one. As for the bumpers, I personnally know how easy the front ones are to crack since my brother'* has damage there which was from bumping a piece of ice while parking on the side of the street. They aren't as flexable as the bonneville ones.
As for the deal, from what you listed it sounds like she is getting a good deal on the car if she gets it. If the cavi is as bad as you state, then she isn't getting hosed there either. I know GP prices here are $5500 for a 98 base gt (no options) with 101k, so if the one she is looking at is 2000+ and less than 50k, she is getting a good deal at $8500.
I'd say get the GP GT.
2007 Saturn Aura XR | 1970 Chevy Blazer 4x4 | 1970 GMC 3/4 Suburban 4x4